mapcomp is package {
    main has type action();
    main() do {
        M1 is hash{"a"->0;"b"->1;};
        M2 is hash{"b"->0;"c"->1;};
        for K1->V1 in M1 do {
            if(not (K1->V2 in M2)) then logMsg(info, "$K1 is not in $M2");
        }
        for K2->V2 in M2 do {
            if(not (K2->V1 in M1)) then logMsg(info, "$K2 is not in $M1");
        }
    }
}